How do I create a reflective white material?
The floor is actually more reflective near the horizon. This effect is called Fresnel and it adds a lot of realism into the scene.

Your shader should look like this:



Instead of controlling the amount of reflection with the Mix Shader Fac input, you control the amount with the value of color of the Glossy node. Black will be no reflection.

As the roughness is 0, this is a PBR dielectric shader. If the roughness would be more than 0, the Fresnel node would have to be upgraded to account for the roughness.

To keep it physically correct, keep the color of the Glossy node white. You adjust the color of the floor with the Diffuse shader.

Here is a comparison between this setup with Fresnel (left) and not physically correct setup with just mixed diffuse and glossy (right):



The right ball should feel un-natural.